1. Field of the Invention
The present invention relates generally to wireless communications and, in particular, to a method for transmitting a Channel State Information Reference Signal (CSI-RS) for a User Equipment (UE) to measure channel quality in a wireless communication system based on a multi-carrier multiple access scheme such as Orthogonal Frequency Division Multiple Access (OFDMA).
2. Description of the Related Art
Mobile communication systems, which were originally designed for providing voice-based services, have developed to wireless packet data communication systems that provide high speed, high quality wireless data and multimedia services. Technology standardization organizations such as 3rd Generation Project Partnership (3GPP), 3GPP2, and Institute of Electrical and Electronics Engineers (IEEE) are working to improve the beyond-3 G communication technologies based on various multi-carrier multiple access schemes. For example, 3GPP Long Term Evolution (LTE), the 3GPP2 Ultra Mobile Broadband (UMB), and the IEEE 802.16m are mobile communication technology standards based on the multi-carrier multiple access schemes for supporting high speed high quality wireless packet data transmission services.
Evolved 3 G communication systems, such as LTE, UMB, and 802.16m, based on the multi-carrier multiple access scheme various techniques including Multiple Input Multiple Output (MIMO) beamforming, Adaptive Modulation and Coding (AMC), and channel sensitive scheduling for improving transmission efficiency. These techniques improve system throughput by concentrating the transmit power of multiple antenna or adjusting the transmit data amount and transmitting data first to the user having good channel quality. Because these techniques operate based on channel quality information between a base station (i.e., evolved Node B (eNB)) and a mobile station (i.e., User Equipment (UE)), the eNB or the UE measures the channel quality, and a CSI-RS is used for this purpose.
Time, frequency, and power resources are limited in a mobile communication system. Accordingly, as resources allocated for a reference signal increase, a traffic channel resource decreases, thereby reducing the amount of data that can be transmitted. In such a case, channel measurement and estimation performance is improved, but the system throughput decreases.
Accordingly, there is a for efficient resource allocation for transmission of the reference signals and traffic channels in order to secure optimum performance in view of system throughput.
In the evolved 3rd generation mobile communication system standards, reference signals are categorized into two categories: Common Reference Signal (CRS) and Dedicated Reference Signal (DRS). A CRS is often referred to as a cell-specific RS or a Common RS in a 3GPP LTE system, and is received by all the UEs within the cell of an eNB. In order to support channel estimation and measurement for transmission with multiple transmit antennas, several reference signal patterns are defined for distinction between antenna ports.
A DRS is an additional reference signal that is transmitted separately from the CRS and is transmitted to a specific UE selected by the eNB. The DRS is also referred to as a UE-specific RS in the 3GPP LTE system and is used for supporting the data traffic channel transmission with non-codebook based precoding.
In LTE-Advanced (LTE-A), which evolved from LTE, a DeModulation Reference Signal (DM-RS) is used for supporting channel estimation of up to 8 layers, in addition to the CRS and DRS. Similar to the DRS, the DM-RS is transmitted in a UE-specific manner, apart from the transmission of CRS.
In the LTE-A system, the downlink signal is transmitted with OFDMA transmission scheme utilizing both frequency and time domains. The downlink frequency band is divided into a plurality of Resource Blocks (RBs), each including 12 subcarriers in a frequency domain, and subframes of which, each including 14 OFDM symbols in a time domain. The eNB performs transmission in a unit of radio resources composed of one or more RBs in a frequency domain and in a subframe in time domain. The resource unit defined by one subcarrier for one OFDM symbol duration is referred to as a Resource Element (RE).
In a Single User-Multiple Input Multiple Output (SU-MIMO) mode or a Multi User-Multiple Input Multiple Output (MU-MIMO) mode, transmission can be performed using multiple layers. For multi-layer transmission, the DM-RS resource is allocated for each layer. The DM-RS resource allocated for channel estimation of one layer is referred to as a DM-RS port in the LTE-A system. Herein, the term DM-RS resource is used interchangeably with DM-RS port.
FIG. 1 illustrates DM-RS patterns designed for use in an LTE-A system.
Referring to FIG. 1, reference number 100 denotes a rank 2 DM-RS pattern in which an eNB transmits DM-RSs for two layers. When transmitting two DM-RSs in the rank 2 DM-RS pattern as illustrated in FIG. 1, the DM-RSs are orthogonally spread with spread factor 2 at positions 101 and 102 and then transmitted in a Code Division Multiplexing (CDM) group. In a similar manner, the orthogonally spread DM-RSs are transmitted at positions 103 and 104. In FIG. 1, the consecutive blue-colored REs carry the DM-RSs. Accordingly, the DM-RSs of two DM-RS antenna ports are Code-Division Multiplexed (CDMed) on the same frequency and time resource.
In FIG. 1, reference number 110 denotes a rank 4 DM-RS pattern in which the eNB transmits DM-RSs for four layers. The rank 4 DM-RS pattern is also spread the DM-RSs with the same spread factor 2 as the rank 2 DM-RS pattern 100, except that additional REs are used for the four DM-RS antenna ports. Accordingly, the rank 4 DM-RS pattern 110 has twice as many REs for DM-RSs as compared to the rank 2 DM-RS pattern 100.
In FIG. 1, reference number 120 denotes a rank 8 DM-RS pattern in which the eNB transmits DM-RSs for eight layers. The rank 8 DM-RS pattern 120 uses the same number of REs as the rank 4 DM-RS pattern 110 for DM-RS transmission. In order to transmit the DM-RSs for eight DM-RS antenna ports with the number of REs same as the rank 4 DM-RS pattern 110, the rank 8 DM-RS pattern 120 orthogonally spreads the DM-RSs with a spread factor 4 at the positions 105, 106, 107, and 108.
In an LTE-A system, the rank of the signal transmitted by the eNB varies depending on a state of the downlink channel. Because the rank of the transmit signal of the eNB varies, the DM-RS pattern also changes depending on the signal rank. That is, the eNB can use the rank 8 DM-RS pattern 120 for the layers having a large number of channels and the rank 2 DM-RS pattern 100 for the layers having a small number of channels. As described above, because the DM-RS pattern is time-varying and the DM-RS port allocated to a UE may also vary, the eNB should notify the corresponding UE of the DM-RS pattern and the DM-RS antenna port to modulate the correct downlink traffic channel.
When the three DM-RS patterns of FIG. 1 are available and a maximum of 8 DM-RS antenna ports are supported, the eNB can notify the UE of DM-RS information using two bits indicating the DM-RS pattern and eight bits indicating the DM-RS antenna port in the form of bit map. That is, in order to notify a UE of the DM-RS resource, a total of 10 bits are used. Assuming that a rank 2 DM-RS pattern, a rank 4 DM-RS pattern, and a rank 8 DM-RS pattern are indicated by 00, 01, and 02, respectively, the eNB can notify the UE that DM-RS antenna ports 1 and 2 are allocated in the rank 4 DM-RS pattern by transmitting the information of 01 and 01100000.
However, using 10 bits of information to notify the UE of the allocated DM-RS resource is relatively redundant, thereby reducing downlink system throughput.
Another problem of the above-described method is that the UE cannot acquire the DM-RS antenna port information for other UEs. That is, the UE can only acquire its own DM-RS antenna port information.
In a wireless communication system supporting MU-MIMO downlink transmission such as an LTE-A system, if transmissions for other UEs on a same time/frequency resource allocated to a specific UE are known, it is possible to implement efficient reception algorithm for the UE. For a receiver operating based on a Minimum Mean Square Error (MMSE), the received determines strength of interference for achieving optimum performance. Further, in order to measure the strength of the interference accurately, the receiver first determines whether or not there is interference. However, the above-described DM-RS resource notification does not provide the UE with any interference-related information.
Accordingly, there is a need to provide a UE with information on whether other UE transmissions are causing interference, in addition to efficient DM-RS resource notification.
In an LTE-A system, an eNB can assign up to 8 DM-RS antenna ports to a single UE. Each antenna port allows channel estimation for one of multiple layers of MIMO transmission of the eNB. The eNB notifies the UE of the allocated DM-RS antenna port using Physical Downlink Control Channel (PDCCH) designed to transmit control information. Because the DM-RS antenna port allocation is required for each layer when the eNB performs MIMO transmission, it is closely related to the MIMO transmission scheme of the eNB. That is, for MIMO transmission for three layers, the eNB transmits the control information on the three DM-RS antenna ports to one or more UEs.